The invention relates to a current limiting device including a substrate or carrier body which is at least partially formed of electrically insulating material. At least one printed conductor configuration being formed of metal-oxide high Tc superconducting material is disposed on the substrate or carrier body and has end pieces for establishing contact with connecting conductors. The invention also relates to a method for producing such a current limiting device. A corresponding device and a method for producing it can be found in European Patent Application 0 523 374 A1.
It is not possible to reliably prevent short circuits and electrical flashovers in electrical alternating-current supply systems. The alternating current in the affected circuit then rises very quickly, i.e. in the first half wave of the current, to a multiple of its nominal value until it is interrupted by suitable protection or switching measures. In consequence, considerable thermal and mechanical stresses due to electromechanical forces occur in all system components that are affected, such as lines and busbars, breakers or transformers. Since those short-term loads increase with the square of the current, reliable limiting of the short-circuit current to a lower peak value can considerably reduce the requirements for the load carrying capability of those system components. As a result, cost advantages can be achieved, for example when constructing new systems and extending existing systems. Accordingly, a replacement of system components with structures having higher load carrying capability can be avoided by installing current limiting devices.
The current rise after a short circuit can be limited to a value of a few multiples of the nominal current through the use of superconductive current limiting devices of the resistive type. In addition, such a limiting device is operational again a short time after disconnection. Thus, it acts like a fast self-healing fuse. It also ensures high operational reliability since it acts passively, i.e. it operates autonomously without previous detection of the short circuit and without active triggering by a switching signal. Resistive superconductive current limiting devices of the type mentioned initially form a superconductive switching path or break to be inserted serially into a circuit. In those configurations, the transition of a superconductive printed conductor configuration from a virtually resistanceless cold operating state below a transition temperature Tc of the superconductive material into a normally conductive state above Tc (so-called phase transition) is utilized. In that case, the electrical resistance Rn which then exists in the printed conductor configuration limits the current to an acceptable amplitude I=U/Rn. The heating above the transition temperature Tc is performed by the Joule effect in the superconductive material of the printed conductor itself when, after a short circuit, the current density j rises above the critical value jc of the superconductive material. The material can already have a finite electrical resistance even below the transition temperature Tc. In the limiting state above the transition temperature Tc, a residual current continues to flow in the circuit containing the current limiting device until an additional mechanical isolator completely interrupts the circuit.
Superconductive current limiting devices being formed of known metal-oxide superconductive materials with a high Tc, having a transition temperature Tc which is so high that they can be kept in the superconductive operating state with liquid nitrogen of 77 K, exhibit a fast increase in electrical resistance when the current density jc is exceeded. Heating into the normally conductive state, and thus current limiting, occurs within a relatively short time so that the peak value of the short circuit current can be limited to a fraction of the unlimited current, for instance from 3 to 10 times the nominal current. The superconductive current path of the printed conductor configuration is in contact with a coolant which is capable of returning it into the superconductive operating state within a relatively short time after the critical current density jc has been exceeded.
A correspondingly operating resistive current limiting device, which can be found in European Patent Application 0 523 374 A1 mentioned initially, contains a wound meander-shaped electrical conductor of high Tc superconductive material (abbreviated as HTS material) as a printed conductor configuration. The printed conductor configuration is machined, for example, from a 5 mm thick plate of the HTS material through the use of slottings. The electrical conductor is self-supporting but can be disposed on a substrate or carrier body to increase the mechanical stability. Its end pieces are constructed as contact pieces to which external connecting conductors can be connected for the purpose of feeding in the current to be limited.
In such a printed conductor configuration having a single printed conductor inserted serially into a circuit, problems occur with respect to the spatial variation of a critical current density jc in the superconductive material. In other words, a corresponding distribution of the critical current Ic(x) is produced, where 0 less than xc3x97 less than 1 and 1 is the length of the printed conductor. The position x having the lowest Ic,min then determines not only the critical current Ic;tot≅Ic,min of the total printed conductor but also is the first one to become resistive during a dynamic limiting process of a short circuit and, consequently, to become resistively heated the most in the limiting phase. The result is that due to such xe2x80x9cweak pointsxe2x80x9d at location x, the maximum switching power is correspondingly low in such current limiting devices.
It is accordingly an object of the invention to provide a current limiting device with a printed conductor configuration of high Tc superconductive material and a method for producing the device, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type, in which the current limiting device can be used to provide a comparatively higher maximum switching power and in which the method provides for the simplest possible production of such a limiting device.
With the foregoing and other objects in view there is provided, in accordance with the invention, a current limiting device, comprising a carrier body at least partially formed of electrically insulating material. At least one printed conductor configuration is disposed on the carrier body and has metal-oxide high Tc superconductive material. The at least one printed conductor configuration has end pieces with a given printed conductor width, for establishing contact with connecting conductors. The at least one printed conductor configuration has n mutually spaced-apart partial conductors connected in parallel between the end pieces and having at least approximately the same conductor length. The n partial conductors each have a printed conductor width reduced at least approximately to an nth part relative to the given printed conductor width of the end pieces.
In fact, in such an embodiment, resistive zones with a lower critical current density jc are exposed to lower current densities in the individual printed conductors but areas with a higher jc which are not yet connected are exposed to greater current densities. This results in a correspondingly lower local thermal loading of the weak points due to resistive power dissipation, on one hand, and switching to the normally conductive state which occurs earlier in time and at lower nominal voltages, on the other hand. The superconductive material is therefore advantageously heated more uniformly, is subjected to lower mechanical stresses and faster recooling after the limiting process is made possible. In other words, the advantages associated with the development of the limiting device according to the invention can be seen in that, in the case of overcurrents, and thus in the limiting case, a current distribution is achieved which reduces the significance of the jc variation and of the weak points.
In accordance with another feature of the invention, the partial conductors of the printed conductor configuration are disposed on the substrate or carrier body in such a manner that the greatest portion of each partial conductor only occupies a surface area of the substrate or carrier body which is allocated to it and which is spatially separate from the surface area allocated to every other partial conductor. This advantageously makes it possible to ensure the greatest possible spatial separation of the individual partial conductors or their current paths which lead to a correspondingly more uniform resistive heating during a phase transition from the superconductive to the normally conductive state.
This effect can be achieved, in particular, when the printed conductor configuration has at least three parallel-connected partial conductors.
With the objects of the invention in view, there is also provided a method for producing a current limiting device, which comprises forming the at least one printed conductor configuration by structuring at least one layer of metal-oxide high Tc superconductive material directly or indirectly applied on the carrier body.
In accordance with a concomitant mode of the invention, the individual partial conductors can be constructed in a relatively simple manner through the use of physical or chemical patterning or structuring processes that are known per se.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a current limiting device with a printed conductor configuration of high Tc superconductive material and a method for producing the device, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.